A High-altitude, Advanced-technology Scanning Laser Altimeter for the Elevation for the Nation Program

In January of this year the National Research Council's Committee on Floodplain Mapping Technologies recommended to Congress that an Elevation for the Nation program be initiated to enable modernization of the nation's floodplain maps and to support the many other nationwide programs reliant on high-accuracy elevation data. Their recommendation is to acquire a national, high-resolution, seamless, consistent, public-domain, elevation data set created using airborne laser swath mapping (ALSM). Although existing commercial ALSM assets can acquire elevation data of sufficient accuracy, achieving nationwide consistency in a cost-effective manner will be a challenge employing multiple low-flying commercial systems conducting local to regional mapping. This will be particularly true in vegetated terrain where reproducible measurements of ground topography and vegetation structure are required for change-detection purposes. An alternative approach using an advanced technology, wide-swath, high-altitude laser altimeter is described here, based on the Swath Imaging Multi-polarization Photon-counting Lidar (SIMPL) under development via funding from NASA's Instrument Incubator Program. The approach envisions a commercial, federal agency and state partnership, with the USGS providing program coordination, NASA implementing the advanced technology instrumentation, the commercial sector conducting data collection and processing and states defining map product requirements meeting their specific needs. An Instrument Synthesis and Analysis (ISAL) study conducted at Goddard Space Flight Center evaluated an instrument compliment deployed on a long-range Gulfstream G550 platform operating at 12 km altitude. The English Electric Canberra is an alternative platform also under consideration. Instrumentation includes a scanning, multi-beam laser altimeter that maps a 10 km wide swath, IMU and Star Trackers for attitude determination, JPL's Global Differential GPS implementation for position determination not reliant on local ground stations, and airborne magnetometer and gravimeter measurements to refine knowledge of the medium-wavelength component of crustal magnetics and the geoid. The laser altimeter utilizes multiple short- pulse (1 nsec) laser transmitters operating at 532 nm, photon counting detectors, and high precision timing electronics to achieve 15 cm range precision per single detected photon. Scanning is accomplished using dual, counter-rotating, transmissive wedges, producing a self-calibrating, overlapping, bow-tie scan geometry that yields a point cloud with a nominal density of three discrete elevation measurements per square meter. Using a single platform to conduct long-range deployments to clear-sky areas and map 16,000 sq km areas per flight mission, complete seamless, consistent national coverage would be accomplished in 3 years. This approach would achieve the goals for the Elevation for the Nation program and provide a capability for on-going, comprehensive mapping to monitor elevation changes associated with natural hazards, human activity, and ecosystem, glacier, ice sheet and snow cover response to climate change.